Roaster Basics: An Explanation of Roast Graphs

Roast graphs may look intimidating at first, but once you understand them, they are a valuable tool. Whether you’re a pro or a home roaster, they can help you achieve better, more consistent roasts; experiment with new profiles; and learn more about this wonderful craft.

So read on for a basic explanation of roast graphs, from what they measure to how pros and home roasters alike can use them.

Why Use Roast Graphs?

There are few feelings more frustrating than realising you can’t recreate that delicious roast you did last month. Consistency across roasts is key. However, as Patrik Rolf, Founder of April Coffee, tells me, that requires recording data – and the more data you have, the better.

“Consistency will only be achieved if you can follow your curves, using the same burner and air settings as the reference,” he says. “When and how you reach a temperature is as important as the temperature in itself.”

This is why we create detailed roast graphs. They act as a reference that allows you to recreate the same roasts time and time again.

Sebastian Brauer, Head Roaster of Elbgold in Germany, also reminds me that “to follow a roast curve is like following the path of understanding coffee better”. Not only will recording data enable you to repeat the roast but it will help you to understand it. You’ll be able to spot how particular elements influence the final flavour profile of your beans, such as charge temp, rate of rise, and more.

And when you want to test a theory, or try something you read about roasting, you’ll have data to compare the results to.

A roast graph will allow you to find even the slight differences in your roasts. Credit: ladyoftheroast

Can Home Roasters Create Roast Graphs?

Roasting graphs don’t just have to be for professionals. While it’s easier to create them with pro equipment and software, anyone who wants to improve their craft can begin tracking data and analysing it.

Colin Hayes uses the Behmor 1600 Plus drum roaster, which offers a “B temp” reading that correlates with environment/air temperature – a roast graph feature that we’ll be looking at shortly. He tells me that he records the B temp every minute to “check against previous roasts of the same bean”. Placing a thermometer inside a home roaster is another option.

Logging and charting air temperature, while not as comprehensive as pro-level graphs, will enable you to control your roasts and follow previously successful curves. So let’s take a look at what these curves mean.

How to Read a Roast Graph

Danny Hall is the developer of Roastmaster, an app designed to record roast data. He divides roast graph data into two types of curves: control curves and reading curves. Control curves are variables that you directly control during the roast, such as the heat settings, airflow, and gas flow. Reading curves are temperature readings. Since the variables are constantly changing, they are recorded as line graphs.

But what reading curves do you need to know? Patrik tells me that the key ones are bean temperature, air/environment temperature, and rate of rise curves – although you can also measure bean colour, air, and gas pressure for even greater insights.

Bean Temperature

As Neal Wilson, the author of open-source data collection app Typica, says in a video tutorial, “one piece of data that people like to have is the temperature of the coffee in the roaster and how that changes over time. This can provide insight into what’s happening both chemically and physically in the bean and aid in comparing roasts or diagnosing quality and consistency issues.” He explains that bean temperature can be read through a thermocouple installed inside the coffee mass.

The bean temperature curve will look a bit like a check mark; once it starts going up (something called the turning point – more on that to come!) it should always continue going up. If not, you risk stalling your coffee and developing bread-like, doughy flavours

Rate of Rise

The rate of rise curve is linked to bean temperature, but there’s a subtle difference: it measures the rate at which bean temperature changes. This will give you far earlier indications of temperature changes and, in turn, allow you more control over the roast. It has a very different shape to the bean temperature curve, rising sharply from zero shortly after the turning point.

Air Temperature

This variable – which is the same one Colin records via B temp on his Behmor 1600 Plus – measures the environment inside the drum. It’s useful to know because much of the heat transfer in coffee roasting is via air. This line will follow a similar shape to the bean temperature curve.

Key Stages on Your Roast Graph

Now we know what the roast graph measures, you can start reading and interpreting these lines. To do so, you want to pay attention to several key points on the graph: charge temp, turning point, first crack, and end temp.

Charge Temperature

This is the temperature of your drum just before you add the coffee. By manipulating this, you can speed up or slow down the rate of rise and, in turn, choose how much acidity to accentuate. You should also pay attention to bean density and processing method when selecting this.

Turning Point

As you add the cold beans to the roaster, the heat inside the machine will dramatically fall before starting to rise again. The point at which it begins to rise is called turning point.

First Crack

One of coffee roasting’s most famous moments, first crack signals that the beans are almost ready. As the beans expand and moisture evaporates, steam develops inside the beans. This steam then forms pressure that cracks the beans open.

End Temperature

As the name suggests, this is the temperature at the end of your roast.

By understanding what’s going on inside the roaster at these key points, you’ll be able to start evaluating the impact of them on your beans. For example, by manipulating charge temp you can speed up or slow down your roast. The duration of first crack can affect body.

One small thing: it’s important to pay attention to the whole roasting process. These are the key points that give the roast structure but they shouldn’t be analysed alone. As Sebastian tells me, “All phases stick together. But points, especially like first crack, need a package of extra care.”

Roasting graphs may, at first, be challenging. There’s a lot of data to collect and understand. However, as you start to work with air temperature, rate of rise, first crack, and more, you will begin to gain real mastery over how your coffee beans develop during roasting. So don’t be intimidated by these charts – start recording those temperatures and see how it helps you as a roaster.